1974 - USGS / Water Resources Council mapped and published the 2-, 4-, 6-, and 8-digit drainages.
1970's-80's - SCS (now the NRCS) mapped Watersheds and Subwatersheds. (Standards were few and dependent on local use or sponsor needs.)
1992 - SCS National Instruction (NI 170-304) released for Hydrologic Unit mapping and digitizing. Provided a comprehensive set of instructions on delineating boundaries and digitizing.
1996 - Comments received from USGS, FS and BLM on working draft. Many comments incorporated into standard, two delineation criteria differences exist.
2000 - Federal Standards for Delineation of Hydrologic Unit Boundaries out for FGDC-Spatial Water Data Subcommittee member review and comment. Even though it is in draft format, states should be using it as the official guideline in 10 and 12-digit hydrologic unit delineation.
2001 - December 6; The Spatial Water Data Subcommittee approved Version 1.0 of the Federal Standards for Delineation of Hydrologic Unit Boundaries.
2004 - October 1; The Spatial Water Data Subcommittee approved Version 2.0 of the Federal Standards for Delineation of Hydrologic Unit Boundaries.
Watershed Definitions (as of March 19, 2018)
Name
Level
Digit
Number of HUCs
Region
1
2
22
Subregion
2
4
223
Basin
3
6
387
Subbasin
4
8
2,318
Watershed
5
10
18,586
Subwatershed
6
12
101,534
Criteria
Scale at 1:24,000 and the base data meets or exceeds the USGS 7.5 Quad Sheets.
Delineation Basis
Hydrologically based
Crosses stream at confluence
May cross at other points if other HUs at same level are upstream
No delineations running down stream centerline
Interagency Standard
Interagency Standard for Delineation of Hydrologic Unit Boundaries
Sponsored by the Federal Geographic Data Committee (FGDC)
Subcommittee on Spatial Water Data with members from the:
